1. Field of the Invention
The present invention relates to novel flame-retardant compositions based on ethylene polymers and to the use of such novel compositions for the protective sheathing of electrical cables. More especially, the compositions of the invention are devoid of halogenated derivatives and can be converted by conventional techniques into shaped articles which have good fire resistance, notably protective insulating covering material for electrical cables.
2. Description of the Prior Art
Improving the fire resistance of compositions containing polymeric constituents is a perennial objective of the manufacturers and converters of such materials. The goal is to markedly reduce the flammability of such materials, as well as their ability to propagate flames. While the use of halogenated flame-retardant agents is well known to this art, this option presents the great disadvantage of resulting in the formation of toxic and corrosive gases upon the combustion or burning thereof. The manufacturers and converters have, therefore, focused on the development of compositions containing metal oxides, hydroxides or inorganic salts in lieu thereof, such as alumina and magnesia hydrates.
In particular, EP-A-326,775 describes flame-retardant polymeric compositions consisting essentially of 120 to 240 parts by weight of hydrated inorganic filler material per 100 parts by weight of the polymeric phase:
(a) 15 to 55 parts by weight of at least one ethylene/vinyl acetate copolymer (A) having a vinyl acetate content ranging from 20% to 50% by weight,
(b) 25 to 62 parts by weight of at least one ethylene/alkyl (meth)acrylate/unsaturated dicarboxylic acid anhydride terpolymer (B),
(c) 8 to 45 parts by weight of at least one copolymer of ethylene and at least one .alpha.-olefin (C) having a relative density ranging from 0.880 to 0.915 and a crystallinity of at least 5%, and
(d) 0 to 5 parts by weight of at least one low-density polyethylene (D).
These prior art compositions comprising 150 parts by weight of alumina hydrate have a limiting oxygen index (determined according to ASTM standard D-2863) ranging from 35% to 37%, an elongation at break (determined according to ASTM standard D-638) ranging from 110% to 125%, and a tensile strength (determined according to ASTM standard D-638) ranging from 11 to 14 MPa. On the other hand, such compositions, which have good fire resistance properties, exhibit a lack of flexibility, reflected in a 1% secant modulus (determined according to NFT standard 54-102 (71)) of at least 200 MPa, which renders them unsuitable for such applications as the production of flexible shaped articles (sheets) and in particular for the sheathing of flexible cables.
Furthermore, GB-A-2,190,384 describes flame-retardant compositions comprising a filler (aluminum trihydrate, magnesium hydroxide) and a polymeric mixture containing (by weight) 10% to 80% of polyethylene or ethylene/vinyl acetate copolymer, 10% to 60% of ethylene/propylene elastomer and 10% to 40% by weight of an ethylene/(meth)acrylic acid copolymer, such compositions having a limiting oxygen index which does not exceed 31.
It is also known to this art, furthermore, that the physical, and especially the mechanical properties (elongation and tensile strength) of compositions of this type are greatly reduced when the filler content increases.